WASHINGTON (CNN) --U.S. Defense Secretary Donald Rumsfeld cast doubt Monday on the authenticity of an audiotaped statement attributed to Osama bin Laden, saying there was no way to tell when the tape was recorded.

Rumsfeld said he did not know whether bin Laden -- the mastermind behind the September 11, 2001, attacks on New York and Washington -- is still alive.

He said he had not heard the tape broadcast Sunday on the Qatar-based television network Al-Jazeera, but he said he had been told the two-minute statement gave no indication of when it could have been made.

"Obviously, there would be many ways that one could easily -- were one alive -- one could easily indicate that they were alive and that the tape had been made recently," Rumsfeld said.

The secretary's comments came a year after U.S. forces launched attacks on Afghanistan. The U.S.-led campaign led to the overthrow of Afghanistan's ruling Taliban, the fundamentalist Islamic militia that allowed bin Laden's al Qaeda network to train and operate from its territory.

Rumsfeld said Taliban leader Mullah Mohammed Omar is "probably" still alive, "but I haven't seen or heard anything hard."

He criticized what he called reporters' "fixation" on bin Laden, saying the war on terrorism "is a problem that is much bigger than one individual."

"Needless to say, we would like to locate him and determine what his circumstance is," Rumsfeld said. "But that's true of 15 or 20 people that we've got high on the list of Taliban and al Qaeda that ... we don't know precisely what's happened to them."

On the tape broadcast Sunday, a speaker said to be bin Laden warns that al Qaeda is preparing attacks on U.S. economic targets.

"I promise you that the Islamic youth are preparing for you what will fill your hearts with horror, and they will target the centers of your economy until you stop your tyranny and terror, until one of us dies," the speaker says. (Full story)

Captured al Qaeda fighters have told U.S. interrogators that bin Laden was wounded in the left hand by the attack on his base at Tora Bora last year. In his last videotaped statement, issued in December, the left-handed bin Laden never moved his left arm and his hand was never shown.

Some intelligence officials believe bin Laden could be hiding in the border area between Afghanistan and Pakistan, a region encompassing several hundred square miles.

"I would come to believe that he probably is dead," Karzai said. "He might be alive. Five months ago, six months ago, I was thinking that he was alive. The more we don't hear from him, the more time passes, there's the likelihood that he is probably dead or seriously wounded somewhere."